This invention relates to a stabilized current source circuit and in particular to a current source circuit employing MOS transistors which supplies a constant current irrespective of the threshold voltage of the MOS transistors.
Various circuits employing MOS transistors have been made in the form of integrated circuits. Among these circuits, filters and integrators need precise current sources as described in "MOS integrated PLL loop filter", 1980 National Conference Record on Communications, The Institute of Electronics and Communication Engineers of Japan, No. 85, for example. In a simple current mitter circuit which has been widely used as the current source circuit, however, the current value unadvantageously varies largely because of nonuniformity of MOS transistor characteristics caused by the fabrication process and because of variation in temperature and in power source.
FIG. 2 shows the principle for configuring a constant current circuit employing MOS transistors. Assuming that an n-channel MOS transistor is used, bias voltage from a bias voltage source 2 is supplied between a gate electrode of an n-channel MOS transistor 1 and a source electrode thereof. As a result, a drain current I.sub.D1 flows through the MOS transistor 1 and the value of the drain current I.sub.D1 is represented as EQU I.sub.D1 =.beta.A(V.sub.GS1 -V.sub.TH).sup.2, (1)
where:
.beta.=.mu.Co/2, A=W.sub.1 /L.sub.1, PA1 .mu.: channel mobility, PA1 Co: gate capacitance, PA1 W.sub.1 : channel width, PA1 L.sub.1 : channel length, PA1 V.sub.TH : threshold voltage.
In the above equation, the nonuniformity of Co, W.sub.1 and L.sub.1 can be limited to ten and several % by sufficiently managing the fabrication process. And the nonuniformity of V.sub.GS1 can also be limited to ten and several % by using a well-known band-gap reference circuit. The variation in each of parameters Co, W.sub.1, L.sub.1 and V.sub.GS1 due to the ambient temperature is negligible. However, the mobility .mu. varies in proportion to the minus one and a half power of the absolute temperature. And the threshold voltage V.sub.TH has fabrication nonuniformity as much as .+-.50% and varies as much as .+-.20% for a temperature change of .+-.50.degree. C. Therefore, the drain current I.sub.D1 largely varies due to the nonuniformity caused by the fabrication process and due to changes in temperature. Furthermore, the variation in power source may cause additional current change. The ratio between the maximum value of the drain current I.sub.D1 and the minimum value thereof amounts to 5 or 6. As a result, it becomes difficult to realize an analog circuit needing a precise current source. In addition, the power dissipation of the circuit varies largely. These are primary factors hampering improvement of analog MOS integrated circuits.
As a stabilized current source circuit employing MOS transistors which is less sensitive to a change in the voltage source and a change in V.sub.TH, a circuit comprising a combination of MOS transistors of different types, i.e., a depletion MOS transistor and an enhancement MOS transistor is known as described in "Constant Current Circuit", Japanese Patent Unexamined Publication No. 51-138848. Since the MOS transistors of different types must be combined, the fabrication process of the circuit becomes complicated. In addition, the relation between magnitudes of currents flowing through three transistors must be set as predetermined. And the gate voltage of a specific transistor must be set to a point where the temperature coefficient is zero. Thus the circuit is subjected to many constraints in its fabrication and design.